The tetragonal (t)-to-monoclinic (m) transformation of ZrO//2 particles randomly dispersed in Al//2O//3 was examined in situ by 1 Mev high-voltage electron microscopy. The residual strain field around tetragonal particles has a principal axis in the C//t direction of t-ZrO//2, as expected from thermal expansion mismatch with Al//2O//3. Two-dimensional analyses indicated that the tetragonal strain axis was replaced by parallel alignment of monoclinic twin plane traces, and hence perpendicular orientation of the monoclinic strain axis, suggesting self-accommodating behavior. The crystallography of the in situ transformation for an approx. 0. 3 mu m particle was elucidated.